Piezoelectric films are known. They are preferably manufactured from polyvinylidene fluoride, have a thickness between 10 micrometers and 100 micro-meters, and a standard width of 300 mm. The piezoelectric film is coated on both sides with a thin, approximately 1 micrometer thick metal layer of aluminum, gold or nickel. Such films are suitable for determining the mechanical load acting on the films, since as a result of the mechanical load a charge variation occurs on the surface of the film, the so-called piezoelectric effect. This serves as measure for the load.
It is known from the publication "Sensor 85 Transducer-Technik, Entwicklung and Anwendung" [Sensor 85 Transducer Technology, Development and Application], Karlsruhe Exhibition Grounds, 21-23 May 1985, Conference Documents, to employ such piezoelectric films having a double-sided metal coating to measure pressures and forces on bodies. The piezoelectric film is bonded onto the body, and the charge variation of the piezoelectric film owing to the effect of the force or pressure is fed to an amplifier. The amplifier output signal then serves as measure for the force acting on the body or for the pressure. It is disadvantageous that the force acting on the body, or the pressure acting on the body, can be determined only as integral value over the surface of the film. It is not possible to determine the precise point of application. However, this is especially important in many investigations of flow. Furthermore, it is disadvantageous that the simultaneous survey of an extensive field is not possible, or is possible only at considerable expense. The sphere of application of these piezoelectric films is therefore limited.
Furthermore, a piezoelectric film is known in which the metal coating is mounted on only one side. The other side of the film is printed with special conductor tracks, for example using the screen printing technique. Although it is possible with this piezoelectric film to measure simultaneously at more than one location, it has the disadvantage, however, that the signal transmitted by the piezoelectric film for the particular location is different given the same stress owing to force or pressure, since the imprinted conductor tracks have different layer thicknesses, and therefore also different line capacities and resistances. In addition, printed piezoelectric films are very prone to breakage in the region of the conductor tracks.
Only quantitative measurements are possible with the piezoelectric films, therefore both with those coated on both sides and also with those coated on one side and printed, since the longitudinal variation of the films reacts to forces in all three axes. With superimposed loads, such as normally occur, it is not possible to filter out any individual components.